In many areas of the world, mobile stations communicate through a cellular network, which comprises fixed base stations and infrastructure links that connect the base stations.
The dependence on base stations and infrastructure communication links between the base stations is not generally desired in emergency cases, such as in areas of war and/or nature disasters, where the base stations may not be functioning or may be overloaded. In addition, some areas of the world are not covered by base stations. Therefore, it is desired to provide a peer to peer network in which communications are established directly between the mobile stations only, without the use of base stations. The peer to peer network is desired to be an adhoc network in which the topology of the network may change often, and therefore measures need to be taken to monitor the topology of the network. The mobile stations of a peer to peer network may be used, for example, by members of a rescue team going out on a mission in a disaster area.
Some peer to peer networks are configured for a star topology in which all or nearly all the mobile stations can communicate directly with each other, or via a single central mobile station, designated as a control station, which allocates bandwidth to the other mobile stations responsive to their requests for bandwidth.
U.S. Pat. No. 6,751,196 to Hulyalkar et al., the disclosure of which is incorporated herein by reference, describes a peer to peer communication network in which a mobile station that serves as a centralized control station is periodically selected responsive to the quality of communications between each pair of mobile stations in the network. Each mobile station monitors signals that it receives from other mobile stations and accordingly determines the quality of the connection with the other mobile stations. The '196 patent suggests using a single intermediate mobile station as a relay station for connections between two mobile stations having a low quality connection.
For situations in which the mobile stations are designed to be distributed over a relatively large geographical area, the use of a star topology requires that the mobile stations transmit at a relatively high power, which results in relatively rapid consumption of the battery power of the mobile stations. For such situations, some peer to peer networks use protocols designed for mesh topologies in which each mobile station communicates directly only with a relatively small percentage of the mobile stations in the network. In order to communicate with remote mobile stations, intermediate mobile stations serve as relay stations. Utilizing a mesh topology moderates power consumption and generally also allows better utilization of bandwidth, as a plurality of mobile stations remote from each other can transmit signals on the same channel concurrently.
In mesh topology networks, a method is required to route messages from a source mobile station to a destination mobile station. A simple method is based on each mobile station that receives a message re-broadcasting the message. This method, however, is very wasteful in bandwidth, as each message may be transmitted many more times than required in order for it to be received by the destination and may be received a plurality of times by many of the mobile stations of the network.
In other methods for transmission in a mesh topology network, each mobile station manages connectivity information on the entire network and thus can route messages to their destination without unnecessary repetitions.
U.S. patent publication 2004/0125776 to Haugli et al., the disclosure of which is incorporated herein by reference, describes a peer to peer network in which each mobile station transmits periodic control messages that include information on how it can be reached, and each mobile station manages a routing table on how each of its neighbors can be reached.
U.S. Pat. No. 6,304,556 to Haas, the disclosure of which is incorporated herein by reference, describes use of a proactive-reactive hybrid protocol for routing within a peer to peer network.
In still other methods, a backbone of nodes is defined for the network. Packets are transmitted from their source to the destination, through the mobile stations belonging to the backbone.
U.S. Pat. No. 6,744,740 to Chen, the disclosure of which is incorporated herein by reference, describes a method for establishing a self-organizing network. Global positioning signals (GPS) are used to determine locations of nodes of the network and form a spanning tree of the network.
U.S. Pat. No. 6,791,949 to Ryu et al., the disclosure of which is incorporated herein by reference, describes a distributed protocol for managing a backbone of a network. Each of the nodes of the network periodically transmits a signaling packet with partial information on the topology of the network.
In some mesh topology peer to peer networks, a channel is available for transmission by all mobile stations, without allocation of the bandwidth of the channel. A mobile station desiring to transmit, checks the channel for an empty slot and transmits in the empty slot. If a plurality of neighboring mobile stations transmit in a same slot, the transmissions are all lost in what is referred to as a contention. Generally, when a contention occurs, the mobile stations identify the contention and organize retransmission of the lost data. The allowance of contentions may result in delayed delivery of data, which is problematic in real time applications. Various methods have been suggested to reduce the rate of occurrence of contentions.
U.S. patent publication 2003/0078062 to Burr, the disclosure of which is incorporated herein by reference, describes a peer to peer network in which a non-allocated synchronization channel is used by mobile stations to arbitrate and select a data channel for communicating.
U.S. Pat. No. 5,887,022 to Lee et al., the disclosure of which is incorporated herein by reference, describes a peer to peer frequency hopping network in which a synchronization channel is used by mobile stations to arbitrate and select a data channel for communicating from a plurality of available channels. The synchronization channel is divided into slots in a manner which reduces the occurrences of contentions.
These methods do not, however, eliminate contentions and therefore other transmission methods were suggested.
U.S. Pat. Nos. 5,719,868 and 6,628,636 to Young, the disclosures of which are incorporated herein by reference, describe a distributed method of assigning slots in a multi-hop network. Each node determines a local slot assignment based on information it receives from its neighbors.
U.S. Pat. No. 6,580,909 to Carro, the disclosure of which is incorporated herein by reference, describes a communication system in which the permission to transmit is passed between the stations of the network as a token in the form of an ordered list including all the stations registered to the network.
These methods are not suitable for real-time transmissions in large mesh networks, as the delay between consecutive transmission sessions of a single mobile station may be larger than acceptable for real time speech transmission.
U.S. Pat. No. 5,943,322 to Mayor et al., the disclosure of which is incorporated herein by reference, describes a peer to peer network which uses a code division multiple access (CDMA) system for transmissions. One of the mobile stations becomes a control station and controls the transmission power in the network.